The present invention relates to an air flowrate measuring apparatus using a heat wire having a temperature-resistance characteristic, which is intended to measure the intake air flowrate into an engine corresponding to the operational conditions of, for example, an internal combustion engine when the same is electronically controlled. The flowrate of the air being measured from the heat-dissipation characteristic of said measuring element, the resultant measurement signal being supplied to an electronic control unit of the internal combustion engine as one of various measurement signals produced for monitoring the operational conditions of the engine.
Monitoring means include engine-revolution detecting means, engine-temperature detecting means, exhaust gas detecting means, etc., and various types of intake air flowrate measuring/detecting means, as means having a direct connection with the operational conditions of the engine.
In a heat wire type air flowrate measuring apparatus, a temperature sensitive element is used which is constituted by a resistance wire having a temperature-resistance characteristic wherein the resistance value varies with the temperature. This temperature sensitive element is fixedly disposed within an intake pipe. To the temperature sensitive element, heating power is supplied to cause it to generate heat. While the heat dissipation of the temperature sensitive element varies depending upon the flowrate of air flowing through the intake pipe, it is detected from the variations in temperature of the temperature sensitive element. A measured air flowrate is represented or expressed by the resultant detection signal. Such an output measurement signal is given, for example, in the form of a pulse-like signal in which there is expressed a time width corresponding to the measured air flowrate.
An electronic control unit for an engine is constituted by, for example, a microcomputer. In this engine control unit, when, for example, an injecting amount of fuel in compliance with the operational conditions of the engine is calculated, a digital arithmetic operation is executed on the basis of detection signals corresponding to such operational conditions. Accordingly, where the signals indicating such operational conditions are obtained in the form of analog data, they should be converted into digital data by a high precision A/D converter.
Accordingly, if the above-mentioned intake air flowrate measurement output signals are produced, as stated above, in the form of pulse-like signals each having a time width corresponding to a measured value, it is possible to supply such output signals to the control unit as digital data by measuring the time width of said pulse-like signal, by using clock signals. Thus, it becomes possible that such output signals are effectively utilized in the control unit as data for calculating the time length to open a fuel injection valve corresponding to the injecting amount of fuel. This eliminates the necessity of providing an input circuit with an A/D converter.
In an output circuit of the air flowrate measuring apparatus designed to generate the above-mentioned pulse-like measurement output signals, usually, a pulse-like measurement input signal is supplied to a base circuit of the transistors. These transistors amplify the input signal and generate the output signals. That is, an output signal which corresponds to an on or off state of the transistor is supplied to the engine control unit. In such an output circuit, however, the output impedance in an on-state of the transistor becomes greatly different from that in an off-state thereof. Accordingly, when the output signal from such an output circuit is supplied to the control unit, it is necessary, in the control unit, to pass the input signal through a filter circuit to remove noises from the input signal. In this case, however, it becomes difficult to equalize the time constant with respect to an increase in the signal passed through the filter circuit, with respect to a decrease in that signal. Particularly when the transistor is off, the output circuit has high impedance on the measuring-apparatus side of a signal transmission line, so that erroneous signals are very likely to be produced due to ignition noises, radio noises, etc., of the engine.